Size Constancy in Monkeys with Inferotemporal Lesions

In a study of size constancy, monkeys were trained with extended practice to choose the larger of two projected stimulus discs independently of distance. When surgical brain lesions were made following pre-operative training, it was found that posterior parietal lesions caused no deficit on this task while inferotemporal lesions caused a severe breakdown from which three out of four animals showed almost no recovery. Mathematical analysis showed that the pattern of performance after inferotemporal lesions could be described as due to a tendency to oscillate between a “correct” and two “incorrect strategies” and that this tendency was also apparent to at least some extent in the unoperated animals. It is argued that the two incorrect strategies might have resulted from a disturbance of size constancy, such that the animal became unable to use both retinal size and distance information in computing physical size and instead used either retinal size or distance information alone. Parallels are drawn between such an hypothetical perceptual disorder and certain clinical disturbances in man.